Revert "Fix a bootloop problem when /data/system/usagestats/usage-history.xml is empty."

This reverts commit 6877837756a8314f8ff681451966f6637f9f9e92.
Fix broken XML parsing idiom.

Code that expected a single top level element in an XML file
was doing something like :

while (type != START_TAG) { next(); }

This would loop forever when the XML being parsed was empty,
where each call to XmlPullParser.next() would return END_DOCUMENT.

bug: https://code.google.com/p/android/issues/detail?id=64173
Change-Id: I7543203e976a8999ae471a6c2d629249a87011bb